Such a tube is similar to the cantilever tube shown in FIG. 3 of U.S. Pat. No. 3,787,780. There the capillary tube is joined to the outer tube by means of a ring-shaped wall, which is firmly attached to the capillary tube on its inside edge and the outer tube on its outside edge. With this design and other familiar ones, such as those which have the capillary tube supported at both end walls of the outer tube, the extremely exact alignment of the two optical elements, which is essential, can be achieved only with great difficulty during production. Only when the units are tested can it be determined whether the alignment is adequate. For that reason, many defective units are inevitably produced. Other familiar designs provide for closing the capillary tube on the open end with a clear glass screen or with a brewster window, with the optical element to be attached to the tube by means of an adjusting apparatus. An optical element so attached can be adjusted during operation of the gas Laser tube, but it is vulnerable to the effects of dirt, shocks and the like. Attaching the optical element by means of a flexible metal tube requires glass-to-metal couplings, as long as the usual glass tubes are retained, making the units expensive and fragile.
In the area of parallel arm laser tubes, which have the outer tube located parallel and adjacent to the capillary tube, methods of adjustment have been developed which are considerably more advantageous, especially for mass production. The optical elements are fixed on the ends of the capillary tube in approximately parallel position. When the device is to be used, fine adjustment is made by means of a mechanism attached to the free ends of the two tubes. The capillary tube is bent slightly until the optical elements are parallel. The chief disadvantages of the parallel arm laser tube are its large size and its instability when heated, because of its asymmetrical design.